Field of the Invention
The invention relates to a filter in general and in particular to a duplex filter for use in connection with a transmitter/receiver.
Radio apparatus which comprises both a transmitter and a receiver using a common antenna usually requires a duplex filter which correctly directs the signal being transmitted and the signal being received. The received signal coming from the antenna must be directed to the receiver without the transmitter causing significant losses. Likewise, the signal being transmitted must be directed to the antenna without the receiver causing significant losses. A duplex filter can be constructed, for example, of helix resonators or of ceramic resonators, and the type and number of resonators with which the filter is realized depends on the requirements set for the filter.
As portable telephones have become ever smaller and lighter and the requirements for the reduction of their power consumption have increased, the requirements set for the filters have become more stringent. Particularly stringent requirements apply to the duplex filter, since, traditionally, it has been the largest filter in the telephone. At the present state of the art this is a difficult task to achieve, since the size of helix filters and dielectric filters is determined mainly by the electrical properties of the individual resonators, such as the resonant frequency, quality factor and the bandwidth of the filter.
In order to be able to construct a filter with the required pass bands while maintaining sufficient attenuation of the stop bands, a certain number of resonators are required for each branch of the duplex filter. Typically, in a cellular telephone the transmitter branch of the filter comprises four resonator circuits, while in the receiver branch it is possible to manage with three resonator circuits. The seven resonator circuits in this example thus determine the physical length of the duplex filter.
It is known that the number of resonators can be reduced by making at least some of them variable. Because, for example, in the cellular telephone system the transmission and reception bands used by the system consist of the bands defining the individual channels, the pass bands of the filters of one or both of the filter branches can be made fairly narrow while nevertheless covering the entire frequency range required by shifting the frequency response of the filter in the frequency plane, which means changing the centre frequency of the filter. In a variable filter the resonance frequencies of the individual resonators are adjusted to provide a filter having the desired frequency response. The final result is that with a smaller number of resonators it is nevertheless possible to cover the desired frequency range.
The adjustment of the individual resonators can be voltage controlled continuous adjustment or current controlled switched adjustment. In switched adjustment the control change shifts the frequency step-wise to the new value. Usually switched adjustment has two positions. The general trend is to use switched adjustment in the filter of the transmitter branch, in the TX filter, because a filter having this type of adjustment is more easily constructed to withstand the high power of the transmitter. A filter having continuous voltage controlled adjustment does not withstand high powers, so that it is suitable for adjustment of the final resonator, seen from the antenna, of the filter of the receiver branch, the RX filter, but the resonator nearest the antenna must be adjusted by switched current control because this resonator circuit must withstand fairly high power since it reflects back the transmitter power while the transmitter is active.
However, this is a considerable disadvantage since current control consumes the telephone's power continuously. In a hand-held telephone as a result of this additional power consumption the standby time falls to almost one third compared with a non-variable duplex filter. Naturally this is not acceptable in a hand-held telephone. Accordingly in state of the art variable filters the resonator nearest the antenna is fixed and not adjustable.